A data access arrangement (DAA) provides an interface between a telephone line and an electronic device that connects to the telephone line. A telephone line to a residence or business can have a common mode voltage in excess of 100V, while most electronic circuit devices typically can not operate at voltages exceeding 5 volts. Accordingly, in order to prevent damage to the telephone network, the DAA must isolate high voltages on the telephone line from any standard electronic device connected to the telephone line. Standard electronic devices are typically powered from the main line, e.g. from wall-mounted A.C. outlets. The data access arrangement, in addition to high voltage isolation, must also transmit and receive electrical communication signals between the central office and the electronic device connected to the telephone line.
A DAA can be constructed using a transformer that operates as a band pass filter and that isolates the electronic device from the telephone line. Alternative DAA solutions use active circuits to isolate the main line powered electronic device from the telephone line. Moreover, those DAAs having active circuits can be powered from main line power or the DAA can be powered from the telephone line itself. Powering a DAA from the telephone line, instead of the main line, would be advantageous because it would reduce the total power requirements of the DAA. In the past, however, DAAs powered from the telephone line have proved problematic.
One problem with powering a DAA by telephone line power is that, after connecting to the telephone line, the DAA must enter a predictable state and it must be ready to transmit and receive when the DAA is connected to certain devices, such as a modem. Secondly, the DC current drain of a line-powered DAA is strictly limited by national and international specifications. For example, telephone line operators typically specify the amount of DC current which can be drawn from their telephone lines during an on-hook state. If a DAA exceeds the national and international specifications, the DAA can not be legally connected to a telephone line. These limitations on current draw have typically prevented telephone line powered DAAs.
Furthermore, telephone line powered electronic circuits, in general, can suffer from a lockup condition if the electronic circuit is powered up by the telephone line improperly. In particular, the digital logic in the telephone line powered electronic circuit can appear as a variable resistor that depends upon the voltage across the telephone line. The variable resistor of the digital logic can appear as a short to the telephone line power, thereby preventing the line powered electronic circuit from being powered at acceptable voltage levels. As a result of being improperly powered, the digital logic enters a lockup condition and the telephone line powered electronic circuit fails to operate properly.
Accordingly, there is a need for an apparatus and method that effectively energizes an electronic circuit, such as a DAA, with telephone line power.